The Swedish Warship Vasa never made it out of Stockholm harbor. It sank on its maiden voyage in 1628, and nearly 400 years later, the ship is suffering a slow, inexorable decay in Sweden’s Vasa Museum.

The mighty vessel has come to symbolize Sweden’s Great Power Period, when the nation became a major European power and controlled much of the Baltic. Though not the largest or most heavily armed ship of its time, the Vasa was formidable: The 226-foot, 64-cannon ship could fire a combined weight of more than 650 pounds of shot from one side. But the maritime pride of Sweden never saw battle — it was too top-heavy, and it capsized and sank after sailing less than a nautical mile.

The Vasa was recovered in 1961 and has been displayed in its own museum since 1990, attracting more than a million visitors per year. But in 2000, signs of deterioration sprang up on the ship’s surface. A new study published July 6 in Biomacromolecules shows that the ship’s wooden hull has significantly weakened, due to decay of the wood’s structural fibers. The Vasa wood is about 40 percent weaker than regular oak wood, and has become very acidic.

“We found a very clear connection between low pH, high degradation, and a large decrease in mechanical strength,” said study co-author Ingela Bjurhager, a mechanical engineer at Sweden’s University of Uppsala.

Bjurhager’s team cut out samples of wood from four different locations on the Vasa to test its properties. They measured the wood’s tensile strength by stretching it in a machine until the wood reached its fracture point, and they analyzed its iron and sulfur content.

Wood gets its mechanical strength from cellulose fibers, and decay of these fibers appears to be responsible for the Vasa’s weakening. The cause of the decay is still being debated, but one theory suggests that iron leaching out of the ship’s metal bolts and fixtures could be combining with oxygen to create a highly reactive substance that’s eating away at the cellulose.

While some microbial degradation took place underwater, the researchers believe most of the decay happened after the ship was brought out of the water. Stockholm harbor provided ideal conditions for preserving a shipwreck: The dark bottom was protected from damaging ultraviolet light, the cold water slowed down chemical processes, and heavily polluted 17th century water prevented an infestation of “shipworm” — a notorious wood-eating parasite.

“Comparatively speaking – Vasa is in an excellent state of preservation compared with most shipwrecks,” said Emma Hocker, conservator of the Swedish National Maritime Museums, in an email.

Soon after the ship was salvaged, it was sprayed with a waxy substance called polyethylene glycol, or PEG, to replace the water inside the wood and prevent it from shrinking. It wasn’t intended to protect against fiber degradation, but it appears to have kept the ship’s surface better preserved than its inner regions.

In 2004, the museum upgraded its climate-control system to keep the relative humidity stable, as fluctuating humidity could lead to changes in the shape and weight of the ship. Efforts are also underway to replace the corroding steel bolts that were inserted in the ship during the 1960s with improved stainless steel ones.

The Vasa does not have an immediate risk of structural failure, the researchers concluded. Still, the ship deforms a few millimeters every year. Given the extent of the wood atrophy, “It’s sort of a little bit too late to do anything,” said Bjurhager, who is focusing instead on preventing further deformation. Her team is currently working on a computer model of the ship so they can design a new support structure.

There may be life in the Vasa yet.“It has a long history with the Swedish people,” Bjurhager said. “People just like it.”